Induction vs. Permanent Magnet Motor Efficiency

Electric motor efficiency significantly influences the industrial, consumer, and automotive sectors. Higher efficiency leads to lower greenhouse gas emissions by reducing power consumption and increasing range between charges — applicable for everything from electric vehicles to power tools. With the ongoing electrification across everyday life, many are curious about which type of motor is best suited to meet modern demands.

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Traditionally, induction motors were the preferred choice due to their availability and proven technology. However, the design of induction motors, which requires a slip between the rotor and stator, inherently limits efficiency. Recent advancements in permanent magnet materials (energy density) and manufacturing techniques allow today’s permanent magnet motor designs to achieve performance and energy efficiency levels unattainable by induction designs.

Let’s delve deeper into both motor designs to support the preference for permanent magnet designs over induction motors. Furthermore, it is essential to understand how Soft Magnetic Composites (SMC) can revolutionize traditional radial flux designs as well as new topologies, driving tomorrow’s designs and performance with reduced heat generation and a more efficient use of copper and magnet materials.

When exploring induction vs. permanent magnet motors, consider the following factors:

• Cost
• Efficiency — torque, core losses, frequency & motor speed control
• Material opportunities
• Application

Permanent Magnet Motor vs. Induction Motor Efficiency

The illustration below displays the layout of both the Permanent Magnet motor (on the left) and the induction motor (on the right). In the permanent magnet design, the rotor features a series of magnets located either internally or externally to the rotor’s outer diameter. The stator is wound with copper wire, creating a magnetic field that interacts with the rotor's permanent magnets, resulting in rotation and torque. In contrast, the induction motor typically has stamped lamination steel for both rotor and stator, where the motor windings are only on the stator, inducing an opposing magnetic field in the rotor, generating rotational torque.

(Comparison of AC induction motor design vs. permanent magnet motor)

Modern high-torque motors, regardless of being permanent magnet or induction designs, utilize three-phase applied current. This three-phase configuration inherently provides better efficiency and is self-starting. If a motor is designed to operate at a fixed rotational speed, the number of stator poles can be adjusted to achieve the desired speed at the standard fixed frequencies of 50 or 60 Hz. In such cases, the laminated induction motor is typically the most commonly chosen option. However, for variable speed motors, a variable frequency power supply is necessary. Although an induction motor could function in this setup, the permanent magnet design offers enhanced performance and increased flexibility.

The intricacies of electric motor design go beyond what is detailed here, but this serves as a solid foundation for those considering their options between induction and permanent magnet motor designs.

Permanent Magnet Motor Efficiency

A permanent magnet motor exhibits higher inherent efficiency than an induction motor, as it eliminates the intrinsic lag of the applied and induced fields. Permanent magnet motors run synchronously with the applied frequency, allowing them to operate at speeds determined by the frequency drive. As the frequency increases, total losses in induction motors surpass those in permanent magnet motors, which can achieve efficiencies of up to 97.5%.

A 50 kW (around 70 HP) permanent magnet motor typically weighs less than 30 lbs. At any given frequency, the rotational speed of the permanent magnet motor consistently exceeds that of its induction counterpart due to the inherent slippage required in the induction design. The synchronous speed can be represented by the following equation:

Ns = 120 * frequency / pole count

(Ns is synchronous speed. Pole count refers to the total number of poles per phase, including both north and south poles)

Currently, permanent magnet motors are utilized in various applications and platforms, including the Ford Mustang Mach-E, BMW, Ultium Platforms, Tesla, high-efficiency variable frequency HVAC motors, battery-powered hand tools, and drones. Notably, every application that requires battery power or high efficiency employs a 3-phase permanent magnet motor.

Induction Motors:

As mentioned, an induction motor operates by the stator winding inducing an opposing current in the rotor, creating a magnetic field. This opposing field results in rotor rotation. The lag between the applied stator current and the generator rotor's opposing field leads to slippage. The maximum speed of an induction motor is calculated using the same equation as that of the permanent magnet motor. However, induction motors inherently require slippage (asynchronous operation). When the slip approaches zero, the torque generated also approaches zero, making synchronous operation of an induction motor impossible. For instance, a two-pole AC induction motor operating at 60 Hz has a synchronous speed of 3600 RPM, but due to a typical 5% speed loss from slippage, the maximum achievable motor speed is about 3420 RPM. This design characteristic confines the maximum efficiency of induction motors to about 90-93%.

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The maximum efficiency of an induction motor ranges between 90% and 93%, whereas that of a permanent magnet motor exceeds 97%. Although a 4% to 7% improvement might appear minimal, consider the operational costs over ten years. Even a slight enhancement in efficiency translates into substantial energy savings and reduced greenhouse gas emissions.

As previously mentioned, a 50 kW (around 70 HP) permanent magnet motor typically weighs less than 30 lbs. In contrast, a standard 75 horsepower induction motor can weigh over 500 pounds. The implications for automobiles are significant—the weight reduction contributes to lower overall vehicle weight.

Cost Vs. Performance

A major consideration with permanent magnet motors is the cost of the magnets. Users of high-energy magnets (like iron neodymium boron) often feel budget constraints. The potential waste from stamping lamination materials further complicates the situation.

Opportunities for powder metallurgy abound in these motors. Permanent magnet motor rotors can be crafted from sintered powder metal, whether adopting an internal or external design. The stator can also be produced using soft magnetic composites. When utilizing the high switching frequencies anticipated, losses in SMCs are lower than in laminated 3% silicon iron, resulting in improved efficiency. To simplify, soft magnetic composites are tailored for high frequencies.

Powder metallurgy presents opportunities to enhance the efficiency of a permanent magnet motor compared to an induction motor. The 3D shape-making capabilities of powder metallurgy facilitate the formation of the stator to entirely encase all wiring in soft magnetic composite, thereby eliminating end turn losses.

These are just a few of the numerous advantages that powder metal—both sintered soft magnetic materials and SMCs—provides.

Induction Vs. Permanent Magnet Motor Efficiency: The Winner Is...

The clear winner here is the permanent magnet motor. By combining permanent magnet motors with a unique topology enabled by Soft Magnetic Composite (SMC) technology, your motor will be lighter, more efficient, have higher torque density, and lower material costs—while simultaneously reducing supply chain complexities and utilizing a sustainable manufacturing process.

If you need assistance designing components to fully harness the potential of powder metallurgy for AC or DC magnetic applications, reach out to us and check our resource hub.